Spin valves using organic spacers and spin-organic light-emitting structures using ferromagnetic electrodes

ABSTRACT

The spacer in a spin-valve is replaced with an organic layer, allowing for numerous applications, including light-emitting structures. The invention demonstrates that the spin coherence of the organic material is sufficiently long that the carriers do not lose their spin memory even in traversing a thicker passive barrier. At least three methods to fabricate the organic spin-valve devices are disclosed, in which the difficulties associated with depositing the ferromagnetic (FM) and organic layers are addressed.

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/472,640, filed May 22, 2003, the entire contentof which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to spin vales and, more particularly,to spin valves incorporating an organic spacer.

BACKGROUND OF THE INVENTION

Spin-valves based on the effect of giant magnetoresistance and tunnelingmagnetoresistance are currently used in high-density magnetic recordingheads and magnetoresistive random-access memories. This type of deviceis based on electrical resistance having two different values; say R₁and R₂ that are dependent on an applied external magnetic field. When amagnetic head is in proximity to the spin-valve device it can change theresistance, or it can change voltage if an electric current runs throughthe device, between the two resistance values R₁ and R₂. The change inelectrical resistance does not involve extra current or voltage; it justreacts to the external magnetic field. A spin-valve can be regarded as aswitch, wherein the application of an external magnetic field does theswitching.

A conventional vertical spin-valve device can be constructed using twothin ferromagnetic layers (each with a thickness of less than 100 nm)and a spacer in between, which can be a metallic or insulating thinlayer (a few nm thick). When the magnetization orientation in the twoadjacent ferromagnetic electrodes is parallel to each other, theelectrical resistance measured perpendicular to the films has value R₁;alternatively, when the two magnetization orientations of the twoferromagnetic films are anti-parallel to each other then the resistanceis R₂, which is different than R₁. The magnetization of the electrodescan be arranged to be parallel or anti-parallel to each other by anexternal magnetic field. The resistance change under the influence ofthe magnetic field has been dubbed magnetoreistance or MR.

SUMMARY OF THE INVENTION

Broadly according to this invention, the spacer in a spin-valve isreplaced with an organic layer. However, the thickness of the layer isnot limited, allowing for numerous applications, includinglight-emitting structures. The invention demonstrates that the spincoherence of the organic material is sufficiently long that the carriersdo not lose their spin memory even in traversing a thicker passivebarrier. At least three methods to fabricate the organic spin-valvedevices are disclosed, in which the difficulties associated withdepositing the ferromagnetic (FM) and organic layers are addressed.

The Advantages of organic spin-valves over existing inorganicspin-valves are many. First, they are less expensive and easier tofabricate than their inorganic counterparts. There are also many morechoices for the materials that make up the organic spacer. As examples,an intermediate layer may be chosen that emits light, changes itselectrical properties upon illumination, can be doped in situ, and issensitive to environmental physical and chemical properties such ashumidity, oxygen level, and other environmental factors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a spin valve constructed in accordance with this invention;and

FIG. 2 is a graph that demonstrates a substantial magnetoresistanceeffect based upon a device constructed in accordance with thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a spin valve constructed in accordance with this invention.There are three important layers in this device, namely the twoferromagnetic electrodes (FM1 and FM2), and an organic layer as aspacer. The device may be built on any suitable substrate material. Theferromagnetic electrodes can be metallic (e.g. Co, Ni, Fe or theiralloys), half-metallic (e.g. ReMnO₃, Re being a rare earth element, orCrO₂), or semiconducting (e.g. GaMnAs). The organic layer canincorporate π-conjugated semiconductor polymers or small molecules (e.g.Alq₃). In the vertical spin-valve devices that we have demonstrated sofar, FM1 was a ferromagnetic oxide, La_(0.7)Sr_(0.3)MnO₃, (LSMO); FM2was a composite layer consisting of Co and Al; whereas the organicsemiconductor was Alq₃. We have fabricated and realized a large MR inthis device (FIG. 2).

Similar spin-valve devices can be realized in planar geometry, in whichFM1 and FM2 electrodes may be the same material but need to havedifferent widths in order to control the magnetization switching in eachelectrode independently. According to this embodiment, we fabricated avertical device based on two different FM electrodes. In addition, wehave also fabricated the spin-valve device and demonstrated itsswitching capability upon the application of an external magnetic field.

In the vertical organic spin-valve devices, the ferromagnetic layers aretypically high-melting temperature material, whereas the organicsemiconducting layer has typically low melting temperature. Accordingly,during the FM electrode deposition process, the deposition temperatureneeds to be much lower than the melting point of the organic materialsif the organic materials have been already deposited. Highertemperatures may evaporate the organic film away or cause intermixingbetween the organic and FM materials that would deteriorate theirinternal magnetization. As a result the intermixing at the FM/organicinterfaces may destroy the magnetoresistance.

In addition, the metallic ferromagnetic electrodes typically oxidizevery fast in air. The oxidized interfaces are detrimental tomagnetoresistance in the final devices. So it is advantageous tofabricate the metallic electrodes together with the organicsemiconductors in vacuum. Sputtering (a common deposition technique) isnot preferred for the metallic electrode deposition if the organic layeris already deposited because the plasma is detrimental to the organicsemiconductors. Thus, the film deposition is preferably carried out invacuum at low temperatures. For some spin-injecting electrodes such asthe ferromagnetic oxides (e.g. LSMO), in-situ deposition is not requiredin fabricating the organic spin-valve since they do not react with O₂ inair. They can be predeposited, cleaned and then introduced into thevacuum chamber prior to the organic and the second electrode deposition.

FABRICATION METHODS

In the following we describe various alternative fabrication methods forthe organic spin-valve.

Method 1

In this method the first layer, FM1 is a predeposited ferromagneticelectrode that is not air sensitive, the organic layer is deposited onFM1 by thermal evaporation at a relatively low temperature, whereas thedeposition of the second ferromagnetic layer, FM2 is done by thermalevaporation with cooled substrates and/or with a cooled region near theevaporation source so that the excess heat can be taken away. Thisensures that the vacuum chamber is at a sufficiently low temperaturethat the deposited organic layer will not evaporate away or intermixwith FM2 at the interface. The thermal evaporation of FM2 can bereplaced with electron-beam evaporation, which typically produces lessheat if the evaporation is from a focused spot.

Method 2

A second method can be independently used or used together with thefirst method. The main idea is to deposit a very thin FM2 layer(thickness of the order of few nm) onto the organic layer so that thehigh deposition temperature will be needed for a relatively short time.We note that a very thin layer (˜1 nm or so) of ferromagnetic materialis already adequate to establish its ferromagnetism at the interface inorder to produce the magnetoreistance. Since a very thin FM2 layer isdeposited then if one starts with relatively thick organic layer, someof it would evaporate away during the FM2 layer deposition, but somewould remain deposited on the first predeposited FM1 layer. To ensurethe device electrical connection and to protect the relatively thin FM2layer, a low melting temperature metal (e.g. Al, Au) is evaporated ontop of FM2.

For demonstrating the organic spin valve we have used the second method.The predeposited FM1 layer was a LSMO ferromagnetic film. We deposited120 nm thick film of the π-conjugated organic molecule AlQ₃ (purchasedfrom Aldrich), and FM2 layer was a 3.5 nm thick of cobalt. A protectivelayer of aluminum was then deposited onto FM2. The magnetizationproperties of the ferromagnetic layers FM1 and FM2 were separatelymeasured by magneto-optical technique (MOKE) and the temperaturedependence of the magnetization and coercive magnetic field wasrecorded.

Method 3

We refer to the third method as a flip/bond method. It can be usedindependently or together with the above two methods. This method workswith either metallic or other ferromagnetic electrodes as FM1 and FM2layers. Both FM1 and FM2 electrodes are deposited first following by anorganic layer deposition in vacuum. Then the electrodes that are alreadycovered with the organic can be taken out of the vacuum chamber. Oneelectrode can be flipped with its organic overlayer facing the otherelectrode with its own organic overlayer. Then the two organic layersare brought together. The electrodes can be aligned and then bonded byheating up to a relatively low temperature to promote adhesion. Thismethods ensures low temperature deposition and no intermixing at themetal/organic interfaces

MATERIALS

The organic vertical spin valve is composed of at least three layers;two ferromagnetic layers and an organic semiconductor layer. Here wemention various possible materials that can be used for this device.

1. Ferromagnetic Layers

These may be metallic, half metallic or magnetic semiconductors.Metallic ferromagnetic may be iron, cobalt, nickel and their composites.The half metallic can be manganites and other magnetic oxides.

2. Organic Semiconductors (π-conjugated) These can be polymers such aspolythiophenes, polyparaphenylenes, polyparaphenylenevynylenes, andpolyfluorenes and their block co-polymers. Also they can be smalloligomers of the above such as 4-thiophene, 6-thiophen, etc, or 3-PPV,such as distyryl benzene, etc. Also they can be small molecules such asporphyrines, AlQ₃, PBD, dendrimers, etc.

In summary, we have fabricated and demonstrated an organic spin-valvedevice. In addition, we have also successfully shown that carriers(electron and/or holes) with aligned spins can be injected into andtransported coherently through π-conjugated organic semiconductor films.This opens up a new field with opportunities to add new functionalitiesto the existing spin-devices or develop entirely new devices.

OTHER APPLICATIONS

(a) The resistance of the organic spin-valves can be tuned. This may becarried out by engineering the HOMO-LUMO levels of the organicsemiconductors relative to the ferromagnetic electrode materials. Thiscan have a great impact in magnetic recording and magnetoresistiverandom access memory technologies.

(b) Since it has been discovered that the organic semiconductorsgenerally have a long spin diffusion length, the organic spacer in thespin-valves can be made relatively thick. This can make the fabricationprocess much more reproducible and reliable in magnetic read heads andmagnetoresistive random-access memory.

(c) The organic layer is a semiconductor; therefore, the conventionalspin-valves can be made active with very interesting possibilities.

-   -   (i) The spin-valve can be fabricated to show a characteristic        I-V response curve of a diode. This can be achieved if the        work-functions of the two ferromagnetic electrodes are chosen to        be very different from each other. This would eliminate or        greatly simplify the complicated and expensive CMOS process used        for isolation transistors in the present magnetoresistive random        access memory.    -   (ii) The organic layer may be chosen to emit light. Then using        ferromagnetic electrodes, FM1 and FM2 to respectively inject        electrons and holes then the organic spin-valve actually is        transformed into an organic light emitting diode (OLED), with        electroluminescence emission upon application of an external        bias voltage. We note that the efficiency of such OLED is        greatly enhanced if the spins of the injected electrons and        holes are controlled by an external magnetic field. In addition        the electroluminiescence emission intensity may be controlled by        an external magnetic field.    -   (iii) The electrical characteristic properties, as well as the        MR value might be changed upon light illumination.    -   (iv) Again the electrical properties may be changed upon in situ        doping in the gas phase.

1. A spin-valve device, comprising: two ferromagnetic electrodes; and anorganic spacer layer between the two ferromagnetic electrodes.
 2. Thespin-valve device of claim 1, wherein the electrodes and spacer arevertically stacked.
 3. The spin-valve device of claim 1, including ametallic ferromagnetic electrode.
 4. The spin-valve device of claim 3,wherein the ferromagnetic electrode is composed of Co, Ni, Fe, or alloysthereof.
 5. The spin-valve device of claim 1, including a semi-metallicferromagnetic electrode.
 6. The spin-valve device of claim 5, whereinthe ferromagnetic electrode is ReMnO₃ or CrO₂.
 7. The spin-valve deviceof claim 1, including a π-conjugated organic semiconductor ferromagneticelectrode.
 8. The spin-valve device of claim 7, wherein theferromagnetic electrode is selected from polythiophenes,polyparaphenylenes, polyparaphenylenevynylenes, and polyfluorenes andtheir block co-polymers.
 9. The spin-valve device of claim 7, whereinthe ferromagnetic electrode is 4-thiophene, 6-thiophen, or 3-PPV, suchas distyryl benzene, or other small oligomer.
 10. The spin-valve deviceof claim 7, wherein the ferromagnetic electrode is a porphyrine, AlQ₃,PBD, dendrimer, or other small molecule.
 11. The spin-valve device ofclaim 1, wherein the organic spacer layer has a thickness of 50nanometers or greater.
 12. The spin-valve device of claim 1, wherein thethickness of one or both of the ferromagnetic electrodes is 100nanometers or greater.
 13. The spin-valve device of claim 1, wherein theelectrodes and spacer layer are configured in a planar geometry.
 14. Thespin-valve device of claim 1, wherein the electrodes are of the samematerial but with different widths to control the magnetizationswitching in each electrode independently.
 15. The spin-valve device ofclaim 1, wherein the electrodes and spacer layer are configured to showan I-V response curve characteristic of a diode.
 16. The spin-valvedevice of claim 1, wherein one or both of the electrodes injectelectrons and holes to generate an electroluminescence emission uponapplication of an externally applied bias voltage.